1. Field of the Invention
The present invention relates to a process for preparing a wound core having a very low core loss, through the use of a very thin silicon steel strip having an axis of easy magnetization in the direction of rolling.
2. Description of the Related Art
The fundamental magnetic concept of an oriented silicon steel derives from the discovery of a crystal magnetic anisotropy of a single crystal of iron in 1926 (see K. Honda and S. Kaya, Sci. Reps, Tohoku Imp. Univ. 15, 1926, 721). The magnetic characteristics of silicon steel have been remarkably improved by significant advances in the development of a cube-on-edge structure by Goss (N.P. Goss, U.S. Pat. No. 1965,559), and currently, the oriented silicon steel is still considered one of the most useful magnetic materials, due to its low energy loss, high magnetic flux density in a low magnetizing force, and very low cost.
Nevertheless, this steel has significant core loss under a high frequency magnetization, and the magnetic permeability is lowered when the sheet thickness is large (0.20 mm or more as an industrial product), and accordingly, the above-described magnetic materials can be utilized only for a magnetization at 50 Hz or 60 Hz.
In 1949, M. F. Littmann disclosed a process for developing a high magnetic permeability and a low core loss in a very thin silicon steel (see U.S. Pat. No. 2,473,156). In the invention of M. F. Littmann, the starting material has a (110)[001]orientation (B.sub.8 =1.74T) and a satisfactory large grain diameter (grain diameter: 0.05 to 10 mm), and is cold-rolled and recrystallized. The above-described silicon steel has characteristics such that, at a sheet thickness of 1 to
5 mils (25.4 to 127 .mu.m), the magnetic flux density (B.sub.8 value) and the core loss at 10 kGs in 60 Hz are 1.60 to 1.71T and 0.26 to 0.53 W/lb (0.44 to 0.90 W/kg), respectively. Nevertheless the above-described material (silicon steel) has a magnetic flux density as low as 1 74T at a maximum, in terms of the B.sub.8 value, which makes it impossible to increase the required magnetic flux density, and thus the size of power source units in electrical machinery and apparatuses cannot be reduced. Further, since the orientation of the grain frequently deviates from the (110)[001] orientation, a generation and extinction of an auxiliary magnetic domain occur, particularly at an excitation of 1.5T or more, and thus the core loss becomes unfavorably very large.
To solve the above-described problems, the present inventors proposed, in Japanese Patent Application No. 63-322030, a very thin silicon steel strip having a very high magnetic flux density and a low core loss at a high excitation. This proposal, however, has a serious problem of how to achieve a lowering of the core loss through a subdivision of the width of a magnetic domain (domain refining treatment), where a wound core is prepared by using a very thin silicon steel strip. For example, even when the core loss of the silicon steel sheet is reduced through the magnetic domain refining disclosed in Japanese Unexamined Patent Publication Nos. 53-137016 and No. 55-18566, in the case of a wound core, the stress relieving annealing of the steel sheet is conducted after fabrication into a core, which causes the local strain introduced into the steel sheet for the magnetic domain refining to disappear, and accordingly, the core loss lowering effect by the magnetic domain refining is also lost.
For example, Japanese Unexamined Patent Publication Nos. 60-255926 and 61-117218 disclose a technique for controlling the magnetic domain wherein the core loss lowering effect due to the magnetic domain refining is not lost even when a stress-relief annealing is conducted after fabrication of the steel sheet into a core, but when the thickness of the product is as thin as 100 .mu.m or less, it is very difficult to apply the above-described techniques. Therefore, a novel technique for controlling a magnetic domain applicable to the production of a wound core through the use of a very thin silicon steel strip, wherein the core loss lowering effect due to the magnetic domain width subdivision is not lost even when stress-relief annealing is conducted after fabrication of a steel strip into a core, is urgently required.